For better and for worse, the internet has exposed us to all the kink there is in the world. Nothing is really taboo anymore when it comes to online fantasies. Yet when it comes to explaining those desires to real-life partners, many people are too shy.
What turns a person on is not necessarily static. We can have pre-conceived notions of what we like, but they may (and likely, will) change and develop over time. Some people develop new kinks and fantasies altogether. Others find the stuff they used to be into doesn't do it for them anymore.
It's almost an oxymoron that the person you're closest with could be the person you're most afraid to share intimate thoughts with. Humans aren't conditioned to talk about "non-standard" sex with anybody. We're even taught to be ashamed of it; something that ultimately leads to sexual dissatisfaction and the yearning to seek out fantasies in secret, completely cut off from your normal life.
Bringing up new or existing kinks – i.e. things that might be out of the ordinary that you'd like to try – can be embarrassing. You might fear being shut down, laughed out, or made to feel perverted. We think we might alienate our partners and even scare them. We worry they'll forever think differently about us and continually ponder our supposed "dissatisfaction" with a current sex life.